1. Field of the Invention
The present invention relates to a method and apparatus for reading magnetization information, and it relates to a novel magnetization detecting technology for reading magnetization information, utilizing change in the polarization state of incident light associated with change in the magnetization state of a recording film and a plasmon enhancement effect.
2. Background Art
With regard to the reading of magnetization information from a conventional hard disk drive (HDD) or a magneto-optical disk, a method for detecting magnetization information in a recording medium is used, by which a device involving the GMR (Giant Magneto-resistance) effect is utilized or change in the polarization state of incident light based on a magneto-optical effect is detected. Meanwhile, as recording units decrease due to the need for higher density, a device capable of detecting a smaller region in the recording medium is demanded. In a magneto-optical disk, while magnetization is detected by utilizing the change in the polarization state of light based on a magneto-optical effect, attempts to minimize the detection region are being made by utilizing, for example, a blue laser with a shorter wavelength or a near-filed effect with an optical probe. For example, Patent Document 1 discloses a technique for measuring magnetization in a recording medium based on a magneto-optical effect. According to the technique, polarized light modulated at a certain frequency is caused to enter an optical fiber probe, and the polarized light is caused to be emitted from a minute opening at the tip of the optical fiber probe so that a polarized light component transmitted through the recording medium is measured.
As a technique for obtaining atomic-level spatial resolution, a technique involving a scanning tunneling microscope (SAT) is known. As a technique for reading magnetization information utilizing a metal probe, for example, Patent Document 2 discloses a method for detecting the direction of magnetization in a ferromagnetic recording layer by utilizing the fact that a tunnel current that flows between a magnetic metal probe and a recording medium including a non-magnetic layer and the ferromagnetic recording layer varies, depending on whether the magnetic metal probe is parallel or anti-parallel to magnetization in the ferromagnetic recording layer. Patent Documents 3 and 4 disclose a method for detecting magnetization information based on a tunnel current by utilizing the fact that electronic states in a three-layer film structure including a ferromagnetic metal layer, a nonmagnetic metal layer, and another ferromagnetic metal layer vary depending on the direction of magnetization. Further, Patent Document 5 discloses a technique for optically detecting the state of light emission of a recording medium, which is excited by disposing a probe electrode opposite to the recording medium and applying a voltage thereto. Such magnetization detecting technologies involving a metal probe offer promising prospects for a technology for reproducing atomic-level information.
While not related to magnetization detecting technologies, it is known that a tunnel current increases when a metal surface in the close vicinity of a metal probe is irradiated with light. For example, Non-patent Document 1 discloses that a large direct current flows by irradiating the gap between a metal surface and a metal probe with light such that plasmon is excited. Patent Document 6 discloses that writing of magnetization can be facilitated by irradiating a metal plate having a gap with a laser light in order to concentrate an electric field in the gap between two metal plates, and by heating a medium disposed immediately below the gap of the metal plates.
Patent Document 1: JP Patent Publication (Kokai) No. 10-325840 A (1998)
Patent Document 2: JP Patent Publication (Kokai) No. 9-134551 A (1997)
Patent Document 3: JP Patent Publication (Kokai) No. 2004-342183 A
Patent Document 4: JP Patent Publication (Kokai) No. 2005-108302 A
Patent Document 5: JP Patent Publication (Kokai) No. 5-250735 A (1993)
Patent Document 6: JP Patent Publication (Kokai) No. 2002-298302 A
Non-patent Document 1: H. Q. Nguyen et al, IEEE Trans. Elec. Dev., 36, 2671 (1989)